Antibody light chains and other secretory proteins are designated for secretion from the cell by the presence of a hydrophobic signal peptide at the amino terminus of the nascent chain. Blobel's signal hypothesis maintains that once the signal peptide region is translated on free ribosomes, it interacts with the endoplasmic reticulum membrane and causes several ribosome receptor proteins to associate into a proteinaceous tunnel spanning the membrane. As translation of the nascent polypeptide chain continues, the signal peptide region is extended into the intracisternal space and the signal peptide is enzymatically cleaved from the growing chain by a membrane-bound signal peptidase. This project will explore the molecular events involved in the transfer of antibody light chains across the microsomal membrane through the use of synthetic peptides from the singal region of prelight chains. A series of doubly radiolabeled peptides will be prepared by solid-phase peptide synthesis to define the structural features needed to bind the signal peptide to the membrane or to permit clevage by the signal peptidase. The site of cleavage will be determined by sequencing the differentially labeled cleavage products. Multimeric forms of the signal peptide will also be used to explore membrane binding and enzymatic cleavage. The membrane-associated ribosome receptor proteins and signal peptidase will be covalently affinity labeled for isolation and characterization using synthetic analogues of the signal peptide. The metabolic fate of the radiolabeled signal peptide will also be explored.